Various means are currently available to separate the components of a multicomponent fluid mixture. For instance, distillation and/or fractionation are frequently used to separate components with different boiling points. However, some fluid mixtures comprise components which have similar boiling points, and in such cases, separation by distillation may be a difficult and an inefficient means to separate these components. Too many contaminants, e. g., unwanted components, also may evaporate along with (or fail to evaporate from) the desired component(s), or the separation may require high energy expenditures due to the recycling through the distillation process that may be necessary to attain a desired degree of separation or purity.
Consequently, adsorption is often preferred as a process for separating components from a multicomponent fluid mixture to obtain relatively pure products, particularly where the components have similar boiling points. In an adsorption process, separation of the fluid components is accomplished because the adsorbent solid material has a physical and/or chemical attraction for one or more of the components of the mixture in preference to other components of the mixture. Although all of the components of a mixture may be attracted in varying degrees to the material, there is a preference engineered into the process, such that predominantly the desired component(s) may be attracted and remain with the material in preference over all others. For instance, polarity of adsorbent and mixture components is frequently used as a means for engineering the adsorptive preference, or relative adsorptivity, into the process. Thus, a hypothetical mixture could comprise two compounds, a polar compound A and a non- or less-polar compound B. Thus, A would have a greater affinity for a polar solid adsorbent than B (and the opposite would be true for a non-polar solid adsorbent).
Although adsorption could be practiced by passing this A-B fluid mixture over a fixed bed of solid adsorbent (allowing A to adsorb and B to remain in the fluid flowing over the adsorbent), a more efficient technique would involve the continuous countercurrent movement of the adsorbent and the fluid mixture. Such a technique is often referred to as a theoretical moving bed (TMB) process. The efficiency can be appreciated by envisioning a single column with (i) downwardly-flowing fluid comprising the hypothetical A-B fluid mixture, and (ii) upwardly-flowing solid adsorbent. A fluid inlet delivers the A-B fluid stream into the column at a point midway between the top and bottom of the column. The A, being polar, is adsorbed by the polar adsorbent and is thereupon carried toward the top of the column, while the apolar B remains in the fluid, flowing down the column. Thus, it can be seen that a relatively short column can quickly achieve a separation that, with stationary adsorbent and downward fluid flow only, might take a substantially longer time. In addition, the column can conveniently be run in a continuous manner by drawing off the fluid, comprising B and substantially no A, from the bottom of the column. Such a stream may be labeled a “raffinate stream” and can be drawn off via a transfer line from the bottom of the column. At the top of the column, a desorbent may be injected by another transfer line, in order to solvate the A from the adsorbent, allowing the desorbent-and-A fluid mixture to be drawn off the top of the column as an “extract stream” carried in an overhead transfer line from a point below the point of desorbent entry, but above the feed entry. Further, the (now-empty) adsorbent may be returned to the bottom of the column via a recycle stream, and the process allowed to run continuously. See, for example, discussion in Kirk-Othmer Encyclopedia of Chemical Technology, Fourth Edition (1991), Vol. 1, at page 582-583 & FIG. 7, which is incorporated herein by reference.
Of course, achieving movement of a solid adsorbent is at best prohibitively difficult in practice. However, the countercurrent flow of a solid adsorbent may be simulated without actually moving the adsorbent—a chief example of which is the so-called “simulated moving bed” (SMB) process. See, for instance, U.S. Pat. Nos. 2,985,589, 4,029,717, 4,402,832, and 4,478,721, as well as Kirk-Othmer Encyclopedia of Chemical Technology, Fourth Edition (1991), Vol. 1, pages 583-596, each of which is incorporated herein by reference. Such processes have been developed for the separation of p-xylene from a mixture of C8 aromatics (UOP Parex™), n-paraffin separation (UOP Molex™), and olefin-paraffin separation (UOP Olex™). Variants have been developed, such as the Toray Aromax™ process for p-xylene separation.
In general, SMB processes involve holding the adsorbent stationary while periodically carrying out the simultaneous downstream advancement of all injection and withdrawal points along the adsorbent system, thereby simulating the upstream (counter-current) “flow” of the stationary solid adsorbent.
The concept may be illustrated by envisioning the following modifications to the previously-described TMB column illustrating separation of A and B. Not counting the theoretical adsorbent recycle stream, the previously-described TMB column is coupled to the following four streams carried in four transfer lines, from top to bottom of the column: (1) an inlet desorbent stream delivered in a first transfer line to the top of the column; (2) an outlet extract stream withdrawn in a second transfer line, just below the desorbent inlet; (3) an inlet feed stream delivered in a third transfer line to the middle of the column; and (4) an outlet raffinate stream withdrawn in a fourth transfer line from the bottom of the column. In the previously-described theoretical moving bed column, each of these four streams remained continuously carried through each of the four stationary transfer lines, while the fluid in the column flowed down through the column (i.e., downward relative to the stationary transfer lines), and the theoretical adsorbent flowed up through the column (i.e., upward relative to the stationary transfer lines). SMB takes advantage of the fact that the same relative motion of the adsorbent (e.g., flowing up through the column) may be obtained by holding the adsorbent stationary while moving the four transfer lines in a downward (e.g., downstream with respect to the flowing fluid) direction. Of course, continuous movement of the transfer lines would be impractical, but periodic simultaneous downward shifting of all transfer lines approximates the same effect.
While this process could in theory be carried out continuously by simple downstream motion on a column of infinite height, this cannot be accomplished in practice. However, the same continuous effect can be achieved by recycling the downward-flowing fluid to the top of the column (allowing the fluid to circulate continually in the same direction through the column), and recycling the motion of the transfer lines such that, when the bottom-most transfer line (e.g., the fourth transfer line in the above illustration) reaches the bottom of the column, it is moved to the top of the column, while the other three transfer lines are simultaneously moved down. The next transfer line (e.g., the third line in the above illustration) is now at the bottom, and at the next movement of the transfer lines, it will move to the top, while the fourth transfer line is correspondingly moved down. It can be seen, then, that the sequence of transfer lines remains constant, even while all the lines are moved along the column. Thus, the sequence of streams carried in those lines likewise remains constant (desorbent inlet-extract withdrawal-feed inlet-raffinate withdrawal, repeating).
Yet, moving transfer lines along a column is itself also difficult in practice. A simpler method involves leaving the transfer lines in place, while sequentially redirecting the fluid streams to the next successive transfer line. This can be accomplished, e.g., by a rotary valve or like device coupled to all four transfer lines connected to the column. See, for instance, U.S. Pat. Nos. 3,040,777, 3,192,954, and 8,168,845, each of which is incorporated herein by reference. The rotary valve is additionally coupled to four other process lines, two of which continuously carry the fluid feed stream and the desorbent stream to the valve, and two of which continuously carry the extract and raffinate streams from the valve. The rotary valve functions on the same principles as a multi-port stopcock, routing the streams (in the order desorbent-extract-feed-raffinate) to and from, as applicable, each of the four transfer lines coupled to the column. The rotary valve may periodically rotate its position, thereby advancing each stream to the next successive transfer line coupled to the column. Further, more than four transfer lines may be used; for instance, 12 transfer lines could be employed, such that two empty transfer lines are between each stream-carrying transfer line at any given time. Each successive shift of the rotary valve would direct each stream to the next successive transfer line.
As is illustrated above, conventional adsorptive separations such as SMB are typically best directed at binary separations (that is, separation of a feedstream into two outlet streams). Thus, conventional approaches employ multiple-step separation to deal with mixtures comprising three or more compounds, each of which is to be separated. That is, in a mixture comprising (in decreasing order of affinity to a polar adsorbent, i.e., most- to least-polar) compounds A, B, and C, a first step would involve separation of polar A from less polar B and C, and a second step would thereafter be necessary to separate the more polar B from less polar C. This separation process can be excessively complex, particularly where the compound of interest is the intermediately-adsorbing B, which cannot easily be cut away from the rest of the mixture in a binary separation. For example, this problem complicates the separation of olefins from complex mixtures such as naphtha and/or the distillate streams from thermal cracking processes such as steam cracking, coking, visbreaking, and the like. Such streams typically contain, in order of increasing affinity to polar solvents or adsorbents, paraffins, olefins, aromatics, and hetero-compounds (wherein the order would be reversed with respect to non-polar solvents or adsorbents). Thus, a conventional simulated countercurrent adsorptive separation would require at least two steps to separate the olefins from such a mixture. A simple single-step adsorptive separation of olefins (or any other intermediately adsorbed compound) from complex mixtures would significantly enhance efficiency of the desired separation.